The vagus nerve branches out to the inner ear, throat, diaphragm, lungs, heart and abdominal organs. We cannot consciously control our heart, kidneys or small intestine, but we can control the muscles of respiration and the muscles of the larynx (that open and close the vocal cords and control the pitch of sound). To facilitate the parasympathetic response in the body (and stimulate the vagus nerve), we would need to exert influence over those two main areas.
WORKING WITH BREATH
1. Deep diphragmatic breathing. If you take fast short shallow breaths your brain perceives it as an invitation to fight or flight activating your sympathetic nervous system; if you slowly let the air in and let it seep out, your brain will take it as an invitation to rest and digest promoting parasympathetic nervous system activation.
TO PRACTICE: Begin each breathing practice by consciously deepening the breath. Then try to maintain the same breathing pace throughout the time you choose to sit. You can intentionally control your respiratory musculature and expand the belly first then the chest on the inhale, and then deflate the chest and gradually contract the abdomen on the exhale. Adding this type of directionality to your breathing patterns helps to slow down the breath and gain more control over the respiratory musculature.
2. Lengthening the exhalation part of the breath (exhale + retain after exhale). Every time you inhale you activate your sympathetic response a bit (heart speeds up a bit, vagus nerve is suppressed); if you hold the air in, that response is accentuated and blood pressure goes up. Every time you exhale you activate the parasympathetic response (heart rate slows down a bit, vagus nerve is active); if you hold the air out for few seconds it will facilitate the parasympathetic activation. To promote parasympathetic activation and vagus nerve stimulation you would need to gradually lengthen your exhale and take a pause after exhale (comfortably).
We work with breath using ratios. We change the relationship between four parts of the breath (inhale-hold after inhale-exhale-hold after exhale) for the purpose of sympathetic/ parasympathetic management. Ratio work helps stimulate the vagus nerve short-term (during the practice) and increase vagal tone long-term (if you do it consistently). Vagal tone indicates the variability between the heart rate on the inhale and the exhale. The greater that variability is, the higher vagal tone you have, which means that your body can easily switch from the fight-or-flight to rest-and-digest mode and visa versa. Higher vagal tone is better for your health; it reflects your sustainability.
TO PRACTICE: Begin with simple breath ratios, for example Inhale for 6 seconds and then exhale for 6 seconds. Then in the course of the practice you can gradually lengthen your exhaltion to 8 seconds and introduce short hold after exhale. During the practice you can gradually extend the ratio to inhale 6 seconds, exhale 8 seconds and hold for 4 seconds. Try to maintain the target ratio for 6 breaths (beginners) or 12 breaths (experienced practitioners) and then gradually return to the comfortable breathing pace.